1. Field of the Invention
The present invention relates to a hydrodynamic bearing structure having hydrodynamic (dynamic-pressure) grooves and to a rotating apparatus having the hydrodynamic bearing structure, a deflection scanning apparatus, a laser beam printer, an image forming apparatus, and a rotating apparatus of hard disk. More particularly, the invention concerns a hydrodynamic bearing apparatus for rotationally supporting a rotary polygon mirror or the like for high-speed scanning with a light beam in laser beam printers, bar code readers, etc., a method of producing the hydrodynamic bearing apparatus, And a deflection scanning apparatus using the hydrodynamic bearing apparatus.
2. Related Background Art
The deflection scanning apparatus used in the laser beam printers, bar code readers, etc. is constructed to implement deflection scanning with a light beam such as a laser beam or the like by use of a rotary polygon mirror rotating at high speed. In the image forming apparatus such as the laser beam printers or the like, the scanning light yielded by the rotary polygon mirror is focused on a photosensitive body on a rotary drum to form an electrostatic latent image thereon, the electrostatic latent image on the photosensitive body is developed into a toner image by a developing device, the toner image is transferred onto a recording medium such as a recording sheet, and the recording medium with the toner image is transferred to a fixing device to heat the toner on the recording medium to fix it, thereby performing printing.
The deflection scanning apparatus of this structure has been increasing its operation speed and accuracy more and more in recent years and, in response thereto, the hydrodynamic bearing apparatus of a non-contact type to permit low-noise and high-accuracy rotation has been and is employed in the bearing part of the rotary polygon mirror.
FIG. 1 is a schematic, cross-sectional view showing a hydrodynamic bearing unit according to a conventional example. The bearing unit has a shaft 102, which is arranged to rotate integrally with a rotary polygon mirror 101 having a plurality of reflecting facets 101a, and a sleeve 103, in which the shaft 102 is fitted so as to be rotatable. The sleeve 103 is integral with a bearing housing 104. Fixed at the lower end of the sleeve 103 is a thrust pad 106 provided with a spherical portion 106a for supporting the lower end of the shaft 102 in the thrust direction. A flange 107 is fixed to the shaft 102 at the upper part thereof. The rotary polygon mirror 101 is pressed against the upper surface of the flange 107 by an elastic press mechanism 108 including a presser spring etc. to form an integral structure therewith, so as to rotate together with the shaft 102.
A yoke 109a holding rotor magnets 109 is fixed to the peripheral part of the flange 107 and the rotor magnets 109 are opposed to a stator coil 110 on a base plate 105 fixed to the bearing housing 104. When the stator coil 110 is energized by driving current supplied from a driving circuit (not shown), the rotor magnets 109 rotate at about 10,000 rpm together with the shaft 102 and rotary polygon mirror 101.
A fluid membrane is created between the sleeve 103 and the shaft 102 with rotation thereof, thereby constituting a hydrodynamic bearing which rotationally supports the shaft 102 in a non-contact state by the dynamic pressure of the fluid membrane. First dynamic-pressure generating grooves 102a and second dynamic-pressure generating grooves 102b are cut with some spacing in between and in the stated order in the upward direction from the lower end of the shaft 102 in the peripheral surface of the shaft 102. Shallow grooves (not shown) forming a hydrodynamic thrust bearing are also provided at the position facing the lower end of the shaft 102, in the upper surface of the thrust pad 106.
With rotation of the shaft 102, a fluid 111 such as oil or the like present in a bearing clearance between the shaft 102 and the sleeve 103 is pulled into the central part of each dynamic-pressure generating groove 102a, 102b to generate a high-pressure region there. Such high-pressure regions work to support the shaft 102 while maintaining the non-contact state in the radial direction between the shaft 102 and the sleeve 103. Because of this non-contact rotation, the above bearing structure has advantages of capability of yielding properties such as lower noise, higher rotation accuracy, etc. than sliding bearings accompanied by metal contact and capability of reducing the size and cost in terms of the number of assembled parts as compared with rolling bearings etc., for example.
There is, however, the possibility that the shaft becomes unable to rotate because of production of wear powder or because of impact or the like upon contact between the shaft and the sleeve during a start period, i.e., during increase of rotating speed, or during a stop period, i.e., during reduction of rotating speed, or at a complete stop when the rotating speed of the shaft is below steady-state values.
There are thus technologies developed to improve hardness, wear resistance, lubrication performance, etc. by placing a nickel plating or an anodic oxide film such as an anodized aluminum film or the like on the bearing surfaces of the shaft and sleeve (Japanese Patent Applications Laid-Open Nos. 63-235719 and 7-27131).
The methods of imparting the wear resistance and lubricating property (lubricity) to the surfaces by forming the plating film or the anodic oxide film on the bearing surfaces of the shaft, the sleeve, etc. according to the above prior art, are effective to a certain extent in preventing the production of wear powder during the start period or during the stop period, but they still have unsolved issues that the step of plating or anodic oxidation is complicated, it is difficult to ensure uniformity of film, and the cost is high because of countermeasures against environmental pollution.
There are demands for much quicker rotation of the rotary polygon mirror in recent years. For example, for rotating the mirror at the speed not less than 15,000 rpm, it is difficult to maintain the wear resistance and lubricity by the above prior art. More specifically, the nickel plating or the anodized aluminum film peels off during the high-speed rotation of the rotary polygon mirror. In addition, the nickel plating or the anodized aluminum film becomes easier to peel off as cumulative rotation time of the rotary polygon mirror increases.
The present invention has been accomplished in view of the unsolved issues of the above prior art and an object of the invention is to provide an inexpensive and high-performance hydrodynamic bearing apparatus capable of maintaining excellent bearing performance throughout a long period while avoiding the production of wear powder by forming an electrodeposition film for improving wear resistance on a fit surface of at least one of a shaft member and a sleeve member, a method for producing the hydrodynamic bearing apparatus, and a deflection scanning apparatus using the hydrodynamic bearing apparatus.
Another object of the present invention is to improve the lubricity as well as the wear resistance.
The present invention thus provides a hydrodynamic bearing structure comprising:
a sleeve;
a cylindrical shaft fitted in the sleeve;
wherein at least one of the sleeve and the cylindrical shaft is rotatable,
wherein a fit surface of at least one of the sleeve and the cylindrical shaft has an electrodeposition film, and
wherein at least one of the sleeve and the cylindrical shaft is provided with a hydrodynamic groove pattern consisting of a plurality of grooves.
The present invention also provides a hydrodynamic bearing apparatus comprising a shaft member and a sleeve member fitted so as to be rotatable relative to each other, and a rotary member arranged to rotate together with the shaft member or the sleeve member, wherein an electrodeposition film in which a silicon-based resin or particles of an inorganic material are dispersed, is formed on a fit surface of at least one of the shaft member and the sleeve member.
The present invention also provides a method of producing a hydrodynamic bearing apparatus, the method comprising a step of electrodepositing an electrodeposition paint in which a silicon-based resin or particles of an inorganic material are dispersed, on a fit surface of at least one of a shaft member and a sleeve member fitted so as to be rotatable relative to each other.
The present invention can provide the hydrodynamic bearing structure in which the electrodeposition film is provided on at least one of the sleeve and the rod-like shaft.
Since this hydrodynamic bearing structure is improved in the wear resistance, the film is kept from peeling off even during high-speed rotation. The structure can also demonstrate high wear resistance, particularly, during increase of rotation speed or during decrease of rotation speed.
Since the electrodeposition step is an easy step, the production cost of the hydrodynamic bearing structure can be lowered.
The film can be formed while readily controlling the film thickness thereof. Therefore, a gap distance of the bearing clearance can be adjusted to a predetermined value.
Utilization of such a hydrodynamic bearing structure allows us to realize improvement in durability and reduction in cost of the deflection scanning apparatus, the image forming apparatus of the electrophotographic method such as the laser beam printers, copying machines, etc., the rotating apparatus of hard disk, and so on.
By providing the electrodeposition film containing the lubricating filler such as the silicon-based resin, the particles of the inorganic material, or the like, on the fit surface of the shaft member or the sleeve member, the wear powder is prevented from being produced upon contact between the shaft member and the sleeve member. The electrodeposition step is easier than the steps of plating, anodic oxidation, etc., and is thus carried out at low cost.
In addition, since it is also easy to control the film thickness of the electrodeposition film, the gap distance of the bearing clearance can be adjusted to a predetermined value by modifying the film thickness of the electrodeposition film on the fit surface of the shaft member or the sleeve member.
By forming the lubricating film by the easy and inexpensive electrodeposition step, the wear powder is prevented from being produced during the low-speed rotation, e.g., during the start period, during the stop period, and so on, thereby realizing the inexpensive hydrodynamic bearing apparatus with excellent bearing performance.
Application of such a hydrodynamic bearing apparatus to the bearing part of the rotary polygon mirror of the deflection scanning apparatus or the like, can contribute to attainment of higher performance and lower cost of the deflection scanning apparatus.